Chemistry deals with the properties and transformations of materials.

Materials are samples of matter; they include a rock, the secretion from

a pituitary gland, the varnish on a table, the odorant from a flower,

cobra venom, a helium atom, and a proton. Chemists observe how ma-

terials change; they coordinate their observations into useful concepts,

and they predict conditions under which specific changes will

occur-often to produce materials never previously observed. Chem-

istry s origins are ancient. Metallurgy, leather tanning, fermentation,

and the manufacture of soap, glass, and pigments were all devised

before man acquired the concepts that now underlie his understanding

of chemical change. Advances in chemical theory and technology

during the past two centuries, however, have been spectacular com-

pared with all earlier progress. The most valuable group of ideas held

by chemists has proved to be the atomic and molecular hypotheses.